Code division multiple access enhanced capacity system

ABSTRACT

A code division multiple access (CDMA) communication system using spread spectrum signaling over a communication bandwidth uses two different signal spectra generated using two different respective spreading code formats, such as NRZ code formatting and staggered Manchester code formatting, for respectively providing nonsplit spectra having a center peak and split spectra having a center null. The spectra are combined during transmission as a CDMA communication signal having a composite spectrum. The use of different code formats produces the composite spectrum of respective center peak and center null spectra that enables increased channel capacity.

STATEMENT OF GOVERNMENT INTEREST

[0001] The invention was made with Government support under contract No.F04701-00-C-0009 by the Department of the Air Force. The Government hascertain rights in the invention.

FIELD OF THE INVENTION

[0002] The invention relates to the field of code division multipleaccess communications systems. More particularly the present inventionrelates to concurrent code formatting of spreading codes in differingformats for use in code division multiple access communications systemsfor increased channel capacity.

BACKGROUND OF THE INVENTION

[0003] Code division multiple access (CDMA) communications have beenused for some time. Typically, transmitted data is formatted and thespectrum is spread using CDMA spreading codes for communicating CDMAspread spectrum communication signals between a transmitter and aplurality of receivers within a null-to-null communications bandwidth. Atransmitted signal includes superimposed spread spectrum signals spreadby respective spreading codes for providing code division access tomultiple receivers. Differing spreading codes provide signal codedivision multiplexing for enabling the respective receivers to acquireparticular respective communication spread spectrum signals among all ofthe transmitted spread spectrum signals of the transmitted signal. TheCDMA communication systems use a particular digital format to format adata stream prior to spectrum spreading and prior to transmission. Thedigital format is also applied to the spreading codes prior to spreadingformatted data. One such digital format is the nonreturn to zero (NRZ)format. Another format is the Manchester format, also known asbiphase-L. In an NRZ CDMA communication system, an NRZ format is used toformat separate data streams into NRZ formatted data streams that arethen spectrum spread by respective NRZ formatted CDMA spreading codesfor transmission to respective receivers. The communication spectrum ofan NRZ formatted and spread spectrum CDMA signal is characterized ashaving a center peak in the communications bandwidth. The communicationspectrum is also referred to as a nonsplit spectrum. In a ManchesterCDMA communication system, a Manchester format is used to format manyseparate data streams into Manchester formatted data streams that arethen spectrum spread by respective Manchester formatted CDMA spreadingcodes for transmission to respective receivers. The communicationspectrum of a Manchester formatted and spectrum spread CDMA signal ischaracterized as having a bandwidth center null of the communicationsbandwidth. This spectrum is also known as a split spectrum.

[0004] Typically, a CDMA system using NRZ formatting has a peak powerspectral density at the center of the frequency band and ischaracterized as a nonsplit spectrum signal. A CDMA system usingManchester code formatting has a power spectral density null at thecenter of the frequency band and is characterized as a split spectrumsignal. Another available digital format is the binary offset carrierformat that also provides a split spectrum of a spread spectrumcommunication CDMA signal. Yet another digital format that provides asplit spectrum of a spread spectrum communication CDMA signal is astaggered binary offset carrier format. The binary offset carrierformat, the staggered binary offset carrier format and the staggeredManchester format are specific cases of the generalized Manchesterformat. Conventional CDMA communication systems typically use NRZ codeformatting. However, CDMA communication systems can also be implementedusing a split spectrum code format, such as the Manchester code digitalformat, staggered Manchester code format, the binary offset carrierdigital format and the staggered binary offset carrier digital format.Manchester formats include all formats formatting an digital inputstream and produces a digital waveform that has one for moretransistions within a symbol time and that are centered about the centerpoint of the symbol time, with a mean amplitude value of zero. Forexamples, Biphase-L has one centered transistion, staggered Biphase-Lhas two symmetric transistions, binary offset carrier has more than onetransistion, and staggered binary offset carrier has more than twotransistions.

[0005] An NRZ CDMA communication system may, for example, have anavailable bandwidth of 200 kHz and have a data rate of 400 bps. Thespreading code chipping rate for the CDMA may be set at 100 kHz so thatthe null-to-null bandwidth for the spectrum spread CDMA signal is 200kHz with a center peak. Channel capacity is the number of communicationchannels, that is, spread spectrum signals, which can be communicatedwithin a given bandwidth. Using NRZ formatting, the channel capacity isabout thirty-eight at a BER of 10⁻⁵. Channel capacity is a valuableresource. Increasing the channel capacity increases the number of usersthat can be served by a CDMA communication system. The NRZ, Manchester,staggered Manchester,binary offset carrier and staggered binary offsetcarrier formatted CDMA communication systems have limited channelcapacities. These and other disadvantages are solved or reduced usingthe invention.

SUMMARY OF THE INVENTION

[0006] An object of the invention is to provide increased channelcapacity in a code division multiple access communication system.

[0007] Another object of the invention is to provide increased channelcapacity in a code division multiple access communication system using aplurality of digital formats.

[0008] Yet another object of the invention is to provide increasedchannel capacity in a code division multiple access communication systemusing spectrum spreading by a pair of digital code formats providingrespective communication signal spectra.

[0009] Still another object of the invention is to provide increasedchannel capacity in a code division multiple access communication systemusing spectrum spreading by a pair of digital code formats respectivelyproviding a communication signal spectrum with a center null and acommunication signal spectrum with a center peak.

[0010] A further object of the invention is to provide increased channelcapacity in a code division multiple access communication system usingspectrum spreading with nonreturn to zero spreading code formattingproducing a communication signal spectrum with a center peak, and with ageneralized Manchester spreading code formatting producing acommunication signal spectrum with a center null.

[0011] Yet a further object of the invention is to provide increasedchannel capacity in a code division multiple access communication systemproviding a composite communication spectrum produced by spectrumspreading with nonreturn to zero code formatting producing a nonreturnto zero communication spectrum having a center peak, and with ageneralized Manchester code formatting producing a generalizedManchester communication spectrum with a center null.

[0012] The invention is directed to a code division multiple accesscommunication (CDMA) system using spread spectrum signaling with atleast two different code formats producing different respectivecommunication signal spectra combined during transmission as atransmitted communication signal having a composite spectrum. A firstgroup of data streams is spectrum spread by a first group of spreadingcodes formatted using a first digital code format. A second group ofdata streams is spectrum spread by a second group of spreading codesformatted by a second digital code format. The formatted data streamsare spectrum spread by respective spread codes using two different codeformats. In the preferred form, nonreturn to zero (NRZ) code formattingand a generalized Manchester code formatting are used on respectivegroups of spreading codes for communicating over respectivecommunication channels. Using NRZ and a generalized Manchester codeformatting, nonsplit and split spectra are produced and superimposedover the communications bandwidth.

[0013] In the broad form of the invention, those communications channelshaving spreading codes formatted by the first code format have a firstcommunication signal spectrum, and those communication channels havingspreading codes formatted by the second code format have a secondcommunication signal spectrum. The first and second communication signalspectra of the transmitted communication signal are superimposed duringtransmitter modulation to provide a composite communication signalspectrum of the superimposed first and second communication signalspectra. Using the two different digital code formats for formatting thefirst and second groups of spreading codes produces two differentcommunication signal spectra forming the composite communication signalspectrum that provides for increased channel capacity. These and otheradvantages will become more apparent from the following detaileddescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of a dual spectrum code divisionmultiple access (CDMA) transmitter.

[0015]FIG. 2 is a block diagram of a dual spectrum CDMA receiver.

[0016]FIG. 3 is a graph of the waveform components of the Manchesterformatted signal.

[0017]FIG. 4 is a graph of the waveform components of the staggeredManchester formatted signal.

[0018]FIG. 5 is a graph of the power spectral densities of communicationspectra using nonreturn to zero (NRZ) formatting and staggeredManchester formatting.

[0019]FIG. 6 is a graph of the CDMA channel capacity as a function ofthe signal to noise (SNR) margin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, a code division multiple access (CDMA) transmitter provides atransmitted communication signal having a dual spectrum over first andsecond sets of communications channels. The dual spectrum is consideredas a composite spectrum having first and second spectra. In thepreferred form, the first spectrum is a split spectrum having a centernull in the communications bandwidth and the second spectrum is anonsplit spectrum having a center peak in the communications bandwidth.The communication signal transmitted by the transmitter is a dualspectrum signal communicated over the null-to-null communicationsbandwidth.

[0021] The first set of communication channels communicate a first N1set of data streams 10. A first data stream of the first set of N1 datastreams of user data is clocked through a first shift register 12 forproviding first shifted user data. The first shifted user data isnonreturn to zero (NRZ) formatted by a first NRZ data formatter 14 forproviding first NRZ formatted data. A first clock generator 16 is usedfor providing a first data clock for clocking the first data streamthrough the first shift register 12 and through the NRZ data formatter14. The clock generator 16 also provides a first code clock to a firstCMDA code generator 18 for feeding a spreading code to a NRZ codeformatter 20 for providing an NRZ spreading code. The NRZ formattedspreading code modulates the first NRZ formatted data using a firstspreading mixer 22 for providing a NRZ spread spectrum signal to a firstmodulator 24.

[0022] For each of the data streams 10, there is a respective firstshifter 12, first NRZ data formatter 14, first CDMA code generator 18,first NRZ code formatter 20, first spreading mixer 22, and firstmodulator 24, of a first communication channel in the first set ofcommunication channels. The clock generator 16 communicates the firstdata clock signal to all of the first shifters 12 and all of the firstNRZ data formatters 14, and communicates the first code clock signal toall of the first CDMA code generators 18 and the NRZ code formatters 20,for synchronized communications.

[0023] The second set of communication channels communicate a second N2set of data streams 26. A second data stream of the second set of N2data streams 26 of user data is clocked through a second shift register28 for providing second shifted user data. The second shifted user datais also nonreturn to zero (NRZ) formatted by a second NRZ data formatter30 for providing second NRZ formatted data. A second clock generator 32is used for generating a second data clock for clocking the second datastream of the N2 data streams 26 through a second shift register 28 andthrough a second NRZ data formatter 30. The second clock generator 32also provides a second code clock to a second CMDA code generator 34 forfeeding a second spreading code to a staggered Manchester code formatter36 for providing a staggered Manchester formatted spreading code. Thestaggered Manchester formatted spreading code modulates the second NRZformatted data using a second spreading mixer 38 for providing a splitspectrum spread signal to a second modulator 40.

[0024] For each of the N2 data streams 26, there is a respective secondshifter 28, second NRZ data formatter 30, second CDMA code generator 34,staggered Manchester code formatter 36, second spreading mixer 38, andsecond modulator 40, of a second communication channel in the second setof communication channels. The second clock generator 32 communicatesthe second data clock signal to all of the second shifters 28 and to allof the second NRZ data formatters 30, and communicates a second codeclock signal to all of the second CDMA code generators 34 and to all ofthe second staggered Manchester code formatters 36, for synchronizedcommunications.

[0025] The first set of data streams 10 and second set of data streams26 are processed through respective communication channels. However,each of the spreading codes of all of the channels is different forcochannel isolation. The clock generators 16 and 32 can be one clockgenerator for providing the same clock signals to the first and secondsets of communications channels. Additionally, the first set of datastreams 10 are data formatted and then modulated by NRZ formattedspreading codes from the first set NRZ code formatters 20, and thesecond set of data streams 26 are data formatted and then modulated by astaggered Manchester formatted spreading codes from the staggeredManchester formatters 36. The first set of modulators 24 and the secondset of modulators 40 provide respective NRZ spectrum spread signals andstaggered Manchester spectrum spread signals to a transmitter combiner42 for combining the NRZ formatted spectrum spread signals and staggeredManchester spectrum spread signals into a composite spectrum signalshaving a dual spectrum. The NRZ formatted spectrum is a nonsplitspectrum signal and the staggered Manchester formatted spectrum is asplit spectrum. Hence, the composite spectrum is a composite of anonsplit spectrum resulting for NRZ code formatting and a split spectrumresulting from staggered Manchester code formatting. The modulators 24and 40 modulate the spread spectrum signals by a carrier signal having acarrier frequency. The composite spread spectrum communication signal isamplified by a high power amplifier 44 and transmitted as a dualspectrum communication signal using a transmitter antenna 46.

[0026] Referring to FIGS. 1 and 2, and more particularly to FIG. 2, adual spectrum CDMA receiver receives the split and nonsplit compositespectrum communication signal as a received communication signal using areceiver antenna 48. The received communication signal is amplified by alow noise amplifier 50 and spectrum despread by a despreading mixer 52.A clock generator 54 is used for providing a receiver clock signal. Theclock generator 54 generates a receiver code clock signal matching thecode clock signal generated in the transmitter. The clock generator 54also generates data clock signals for formatting and data detection. Inone form of the invention, a user control 57 is used for selecting thetype of code formatting. In the preferred form, NRZ and staggeredManchester code formatting is respectively used for formatting the firstand second sets of spreading codes. For a particular channel, and hence,for a particular CDMA code, the received communication signal isdespread using a spreading code formatter corresponding to one of theNRZ or staggered Manchester code formatters 20 or 36 used to spectrumspread one of the formatted data streams 10 or 26 in the transmitter.The user control 57 controls the selection of the code format. Aselectable NRZ or staggered Manchester formatter 58 is selectable to beeither an NRZ code formatter or a staggered Manchester code formatterand is clocked using the code clock signal from the clock generator 54.

[0027] A receiver CDMA code generator 56 generates a replica spreadingcode for the respective communication channel. The replica spreadingcode and the code formatter 58 in the receiver are identical to thespreading code and the code formatter used in the transmitter for thesame communication channel. The CDMA code generator 56 generates a CDMAcode that is fed to the receiver code formatter 58 for providing aformatted code to the despreading mixer 52 that then despreads thecommunication signal for providing a despread signal. The despreadsignal is communicated to a conventional code and carrier tracking loop60. The code tracking loop 60 provides a clock error signal to the clockgenerator 54 for adjusting clock timing for the despreading code formaintaining code tracking. The carrier tracking loop 60 provides acarrier replica to a carrier demodulator 62 for demodulating thedespread communication signal into a carrier demodulated data stream.The carrier demodulated data stream from carrier demodulator 62 is fedto a bit synchronizer 64 generating a bit timing signal that is fed to adata detector 66 for synchronized clocking of the demodulated datastream into a replica data stream 68. Bit timing may also be generatedfrom the tracking loop 60. The data clock signal from the clockgenerator 54 is received by the data detector 66 for synchronizing thereplica data stream 68. The replica data stream 68 is a replica of thedata stream 10 or 26 spectrum spread by the spreading code. In thismanner, the receiver can be used to receive either an NRZ or a staggeredManchester code formatted CDMA signal of the composite communicationsignal respectively having either a nonsplit spectrum or a splitspectrum.

[0028] The preferred form of the receiver is a code format selectablereceiver. The transmitter can be adapted to change the code format for arespective channel by feeding a data stream into either an NRZ orstaggered Manchester code formatted communication channel. However, itshould be apparent that the receiver could be a fixed code formatreceiver using either NRZ or staggered Manchester code formatting, butnot both, without the use of the user control 57, and without aselectable formatter 58. The formatter 58 is then either a fixed NRZ ora fixed staggered Manchester code formatter. In either case, the datastreams can have the same data formatting, such as NRZ data formattingby NRZ data formatters 14 and 30.

[0029] Referring to FIGS. 3 and 4, Manchester and staggered Manchestercode symbol waveforms are respectively shown for showing that thestaggered Manchester code symbol waveform is a replica of the originalManchester code symbol waveform but staggered, that is, shifted, in timeby a quarter of the code symbol time τ. The staggered Manchester codeformatting is done by staggering the underlying square wave signal by aquarter of the square wave cycle compared to the underlying square waveof the Manchester formatted code signal. As a result of this staggering,the first quarter of the square symbol gets moved to the last quarter ofthe square wave symbol as shown in the FIG. 4. It can also be seen fromthis figure that the antisymmetry in the waveform shape between thefirst and the second half of the code symbol with a Manchester codeformat is changed in the case of staggered Manchester code format to asymmetrical relationship between the two halves of the code symbolwaveform.

[0030] Referring to all other Figures and more particularly to FIGS. 5and 6, the communication channel signals using NRZ code formatting orstaggered Manchester code formatting have respective nonsplit and splitspectra occupying the same null-to-null communication bandwidth, thatmay be, for example, 200 kHz with a data rate of 400 bps. The codechipping rate for the NRZ code formatter in a CDMA system may be a 100kHz chipping rate for providing the null-to-null bandwidth for the NRZcode formatted CDMA signal. With only NRZ code formatted CDMA signaling,the capacity of fifty CDMA channels is achieved with an available linkmargin of 6.0 dB at a BER of 10⁻⁵. Under the same conditions, but withadded staggered Manchester code format signaling at 50.0 kHz, forproducing a dual spectrum CDMA signal, the overall channel capacity isincreased. The communication channel has overlapping nonsplit and splitspectra respectively provided by the NRZ code formatting and staggeredManchester code formatting. The total channel capacity is the sum of theNRZ code formatted channels and the staggered Manchester code formattedchannels. The sum total is improved to sixty nine channels, which is a38% improvement in the channel capacity over a conventional CDMA systemusing only NRZ code formatting generating a nonsplit spectrum.

[0031] Power spectral densities for the NRZ and staggered Manchestercode formatted signals, filtered with a 6th order Butterworth filterhave a cutoff of 100 kHz, as is shown in FIG. 5. A 19% to 48% capacityimprovement is practicable using a combination of NRZ code formattingand staggered Manchester code formatting within a given frequencybandwidth for link margins of 3-12 dB. With a modest increase in CDMAsystem complexity, a CDMA system can obtain increased channel capacityusing different code formatters for providing different overlappingpower spectral densities within the null-to-null communicationbandwidth. In the preferred form, NRZ code formatting generates nonsplitspectra, and, staggered Manchester code formatting generates splitspectra of the dual spectrum CDMA communication signal.

[0032] The present invention is directed to a dual spectrum CDMAcommunication system using two different code formats for providingrespective spectra overlapping within a communication bandwidth. Therespective spectra share the same bandwidth with minimal crossinterference due to one spectrum having a center peak and the otherspectrum having peaks away from the center, for effective bandwidthsharing within the same communications bandwidth. The dual spectrum CDMAcommunication system offers increased channel capacity. It should now beapparent that a mix of transmitters and receivers could operate as partof a complete communication system communicating both split and nonsplitspectrum signals. For example, one group of transmitters or satellitescould transmit split spectrum signals while another group oftransmitters or satellites could transmit nonsplit spectrum signals, allof the signals communicating within the same CDMA communicationsbandwidth. Those skilled in the art can make enhancements, improvements,and modifications to the invention, and these enhancements,improvements, and modifications may nonetheless fall within the spiritand scope of the following claims.

What is claimed is:
 1. A system for communicating a first formatted datastream and a second formatted data stream through a dual spectrum signalover a communication bandwidth, the system comprising, a first codeformatter for formatting a first spreading code into a first formattedcode, a first spreader for spectrum spreading the first formatted datastream by the first formatted code into a first spread spectrum signal,a second code formatter for formatting a second spreading code into asecond formatted code, a second spreader for spectrum spreading thesecond formatted data stream by the second formatted code into a secondspread spectrum signal, and a modulator for combining and communicatingthe first spread spectrum signal and the second spread spectrum signalinto the dual spectrum signal, the first spread spectrum signal having afirst spectrum over the communication bandwidth and the second spreadspectrum signal having a second spectrum over the communicationbandwidth, wherein, the first code formatter is an NRZ code formatter,and the second code formatter is a staggered Manchester code formatter.2. The system of claim 1 wherein, the first spectrum is a nonsplitspectrum with a peak within the communication bandwidth, and the secondspectrum is a split spectrum with a null within the communicationbandwidth.
 3. The system of claim 1 wherein the system is a codedivision multiple access system.
 4. The system of claim 1 having a firstreceiver for spread spectrum despreading the first spread spectrumsignal and the second spread spectrum signal, the first receivercomprising, a first replica code formatter for formatting a firstreplica spreading code into a first replica formatted code, the firstreplica spreading code being a replica of the first spreading code, anda first despreader for spectrum despreading the first spread spectrumsignal by the first replica formatted code into a first despread signal.5. The system of claim 1 having a second receiver for spread spectrumdespreading the second spread spectrum signal and the second spreadspectrum signal, the second receiver comprising, a second replica codeformatter for formatting a second replica spreading code into a secondreplica formatted code, the second replica spreading code being areplica of the second spreading code, and a second despreader forspectrum despreading the second spread spectrum signal into a seconddespread signal.
 6. The system of claim 1 further having a firstreceiver and a second receiver, the first receiver comprising, a firstreplica code formatter for formatting a first replica spreading codeinto a first replica formatted code, the first replica spreading codebeing a replica of the first spreading code, and a first despreader forspectrum despreading the first spread spectrum signal into a firstdespread signal, and a detector for detecting the first data stream forthe first despread signal, and the second receiver comprising, a secondreplica code formatter for formatting a second replica spreading codeinto a second replica formatted code, the second replica spreading codebeing a replica of the second spreading code, and a second despreaderfor spectrum despreading the second spread spectrum signal by the secondreplica formatted code into a second despread signal.
 7. The system ofclaim 1 further comprising, a first replica code formatter forformatting a first replica spreading code into a first replica formattedcode, the first replica spreading code being a replica of the firstspreading code, a first despreader for spectrum despreading the firstspread spectrum signal into a first despread signal, a second replicacode formatter for formatting a second replica spreading code into asecond replica formatted code, the second replica spreading code being areplica of the second spreading code, and a second despreader forspectrum despreading the second spread spectrum signal into a seconddespread signal, wherein, the first code formatter is an NRZ formatter,the first spread spectrum signal is a nonsplit spectrum signal, thefirst spectrum is a nonsplit spectrum having a center peak, the secondcode formatter is a staggered Manchester formatter, the second spreadspectrum signal is a split spectrum signal, the second spectrum is asplit spectrum having a center null, the first replica code formatter isan NRZ formatter, and the second replica code formatter is a staggeredManchester code formatter.
 8. The system of claim 1 further comprising,a first replica code formatter for formatting a first replica spreadingcode into a first replica formatted code, the first replica spreadingcode being a replica of the first spreading code, a first despreader forspectrum despreading the first spread spectrum signal into a firstdespread signal, a second replica code formatter for formatting a secondreplica spreading code into a second replica formatted code, the secondreplica spreading code being a replica of the second spreading code, anda second despreader for spectrum despreading the second spread spectrumsignal into a second despread signal, wherein, the first code formatteris in a transmitter, the first spread spectrum signal is a nonsplitspectrum signal, the second code formatter is in the transmitter, thesecond spread spectrum signal is a split spectrum signal, the firstreplica code formatter is in a first receiver, the second replica codeformatter is in a second receiver, the first formatted data stream iscommunicated between the transmitter and the first receiver, and thesecond formatted data stream is communicated between the transmitter andthe second receiver.
 9. The system of claim 1 further comprising, afirst replica code formatter for formatting a first replica spreadingcode into a first replica formatted code, the first replica spreadingcode being a replica of the first spreading code, a first despreader forspectrum despreading the first spread spectrum signal into a firstdespread signal, a second replica code formatter for formatting a secondreplica spreading code into a second replica formatted code, the secondreplica spreading code being a replica of the second spreading code, anda second despreader for spectrum despreading the second spread spectrumsignal into a second despread signal, wherein, the first code formatteris an NRZ formatter, the first spread spectrum signal is a nonsplitspectrum signal, the second code formatter is a staggered Manchesterformatter, the second spread spectrum signal is a split spectrum signal,the first replica code formatter is an NRZ formatter, the second replicacode formatter is a staggered Manchester code formatter, the first codeformatter and the second code formatter are disposed in a transmitter.10. The system of claim 1 wherein, the staggered Manchester format is astaggered Biphase-L format.
 11. The system of claim 1 wherein, thestaggered Manchester format is a staggered binary offset carrier format.